I still see a lot of people recommending power supplies for builds based on the "recommended" wattages put up by Intel/AMD/Nvidia or who ever. For the most part, those will help you, but too many people take that as the minimum wattage for the graphics card or single piece of hardware alone. That is incorrect and leads to people buying stupidly high wattage power supplies when they can run perfectly fine, with overclocking headroom, on half the wattage. To properly calculate required wattage, you need not use over-estimating online calculators, as they are also just a rough guess.

To calculate your power supply requirements and wattage/amperage you will need, we need a few things:

The components you want to buy and their TDP's (Thermal Design Power)

Some basic knowledge of electricity and maths

A calculator that can add, divide, subtract and multiply

A budget for a power supply

Wattage and Amperage

Let's begin by taking the following components at their stock specifications and adding their maximum TDP's together for total power draw:

Intel i7 2600K (95W)
GTX 570 (225W)
Samsung F3 1TB (25W)

Those are maximum values.

That gives a total of 345 Watts, at maximum load. That means you could run a system with a 2600K and GTX 570 on a quality 400W power supply, and only at bone stock settings. However, a 400W power supply won't have the full 400W available on the one rail. It will be split between the 3.3v, 5v, and 12v rails. For today's computers, the 12v rail is the most important and thus has the most power available compared to the other rails. I have also not included things like Motherboard and RAM power usage because they are hard to find, and not listed on manufacturer sites. Because of this, and overclocking, you want headroom when buying a powersupply. How much depends on how far you intend to overclock or how much hardware you will be packing into your system. Obviously, the more hardware, the more power it will draw.

But this only gives us a wattage. Sometimes, multiplying the amperage by 12 will give you a smaller number than what is on the wattage sticker. Likewise, diving the wattage by 12 may give you a smaller total amperage than what the sticker tells you. It is very important to calculate both, and use the one that gives you the smaller number. Why? It's because you'd rather be safe than sorry when calculating your power draw. Moving on, 345 divided by 12 gives us 28.75. This would be our maximum amperage draw. Compare this to your chosen PSU's specifications and judge accordingly.

Efficiency, and why it matters

Moving on to efficiency. Let's take that previous 345W, and say that the power supply powering those parts is 50% efficient. This still means your system is only drawing 345W from the power supply, but the power supply would be drawing 690 Watts from the wall socket. Why is this important? It's important because this will affect your power bill! This is why you want a power supply that has high efficiency, and because different systems have different roles, you will want a different power supply for a system that will be folding 24/7 and seldom idle, compared to an all-around home system that will be browsing, doing office work and playing games. Let's start with the folding example and the previous parts/wattages listed, and choose a power supply.

Because the system would be on 24/27 and at maximum load, drawing 345W, we would choose a power supply that would be 80Plus Gold/Platinum rated at 450 watts or so. For example, the Seasonic X-460 Fanless Gold. Why? The power supply will have a small amount of headroom, giving us higher efficiency. This will help lessen your power bill compared to running an 850W 80Plus Silver rated power supply. Why? The power supply would be under a lower load, leaving us with lower efficiency, which will mean it draws more power from the socket and will increase your power bill.

Let's take that same system and wattage and assume it's a home system this time. It's almost the same deal, except that the system would be idling most of the time, but would still reach maximum load on games and perhaps benchmarks. Efficiency will once again play a part in this. In this position, something like a Seasonic X-650 80Plus Gold will be more ideal for the home user/gamer/benchmarker. For one, you will still have decent efficiency at idle/low loads, you will have good efficiency at full load, and you will also have headroom for overclocking or another graphics card.

Conclusion

In conclusion, before choosing a power supply and parts, it is utterly important to properly calculate your needed wattage and amperage. The best case scenario of a bad/wrong power supply is having an AX1200 for a system that draws 200W. Your system will work perfectly fine, but efficiency will be woeful. In the reverse scenario, having a system that needs 600w on a 450W power supply, you are likely to end up with dead hardware. While I have not touched on quality vs. junk power supplies, it is very important to choose a quality one for your peace of mind and for your wallet. You don't want to be replacing your entire system because your yum cha power supply blew up into smoke. Please take the time to research your power needs carefully!

If you buy a power supply and it's 50% efficient you should throw it into the garbage can. The only time a PSU should ever be 50% efficient is if you have some crazy ass atom build that draws like 50W and you couldn't find an ATX PSU smaller than 600W for some weird reason. Even then I'm not sure.
Not to mention that the difference between a 460W gold rated power supply and an 850W silver rated power supply running a 345W system load will be something like 10Â¢/month (80Plus silver runs 88% efficient at 50% load, 80Plus gold runs 87% at 100% load). The reason you don't buy the bigger power supply is because it costs more and is a waste of your money, or you can use the same money to get a higher quality line of power to your components if you care about that, not because it saves you a few cents on your power bill.

As for the rest, it's not necessarily bad advice, but it can be a little misleading in its simplification, you don't just look at wattage and amps in a power supply, there are tons of over-rated power supplies out there, and you need to check reviews and/or with people here to make sure that you're not buying something that SAYS it can do 850Watts but can really barely deliver 450Watts (like the TT Black Widow for example) while remaining in spec.

The simplest way to find what your system needs is to check hardware reviews, especially GPU reviews, as they often test what the system actually draws under a realistic load with your card. These will be wall draws, so that gives you the extra headroom to be safe. Then check for PSUs capable of delivering that much power and get reviews of them. Easy, done.Edited by ocpokey - 5/20/11 at 5:35am

If you buy a power supply and it's 50% efficient you should throw it into the garbage can. The only time a PSU should ever be 50% efficient is if you have some crazy ass atom build that draws like 50W and you couldn't find an ATX PSU smaller than 600W for some weird reason. Even then I'm not sure.
Not to mention that the difference between a 460W gold rated power supply and an 850W silver rated power supply running a 345W system load will be something like 10Â¢/month (80Plus silver runs 88% efficient at 50% load, 80Plus gold runs 87% at 100% load). The reason you don't buy the bigger power supply is because it costs more and is a waste of your money, or you can use the same money to get a higher quality line of power to your components if you care about that, not because it saves you a few cents on your power bill.

As for the rest, it's not necessarily bad advice, but it can be a little misleading in its simplification, you don't just look at wattage and amps in a power supply, there are tons of over-rated power supplies out there, and you need to check reviews and/or with people here to make sure that you're not buying something that SAYS it can do 850Watts but can really barely deliver 450Watts (like the TT Black Widow for example) while remaining in spec.

The simplest way to find what your system needs is to check hardware reviews, especially GPU reviews, as they often test what the system actually draws under a realistic load with your card. These will be wall draws, so that gives you the extra headroom to be safe. Then check for PSUs capable of delivering that much power and get reviews of them. Easy, done.

Thanks for the criticisms, they are appreciated. I wrote that in the mind that everyone reading it would know the difference between a good and bad power supply. Yes you should throw a 50% efficient PSU in the bin, but I used it as an example to show what efficiency does. This is why I used Seasonic PSU's as examples because they are very good quality. There are many guides telling people what's good and what's crap. I wrote this telling people how to accurately calculate what their components will draw and that will tell them which powersupply is right for them, if they know only to buy the good quality ones (Antec/Corsair/Seasonic/Silverstone/XFX + more) - Many people preach that for the right reasons.

I also live in a house of five people. There are three computers and every bit counts. Energy isn't as cheap here and it's also tipped to go up by 30%. We get $1000 electricity bills in winter. So yeah. I wanted to keep the whole thing simple so it was easy to read and understand for your average user. It ain't always cheap and that 10c for you might equal a few dollars for us.

I wrote another guide on another forum that is all about good and bad power supplies and why they are so, and which you should choose. I won't link it but if anyone wants to take a look, throw me a PM. I haven't updated it in a while though.

Thinking of average power consumption, you are probably right. Engineering design states that your normal power usage should be half/50% of the power supply rating considering you have a 100% efficient machine. Power supply cannot run 100% of its load 100% of the time. You need to leave a room for power consumption spikes and so one. (Think of your car running 6000RPM at any given time)

Thinking of average power consumption, you are probably right. Engineering design states that your normal power usage should be half/50% of the power supply rating considering you have a 100% efficient machine. Power supply cannot run 100% of its load 100% of the time. You need to leave a room for power consumption spikes and so one. (Think of your car running 6000RPM at any given time)

Interesting argument though.

It can if it's a quality one Engines and power supplies are different beats, but you make a valid point. I'm really just trying to get people away from seeing "550W psu recommended" and taking it as needing 550W for the single component, when it's for the entire system. It still happens too often.